The limit value is 125 µg SO2/m3 as a daily average, not to be exceeded more than three days in a year and to be met by 2005. Over the years 1997-2009 the total population for which exposure estimates are made, increased from 85 to 100 million people due to an increasing number of monitoring stations reporting under the Exchange of Information Decision. Year-to-year variations in exposure classes are partly caused by the changes in spatial coverage. Only urban and sub-urban background monitoring stations have been included in the calculations. Data for Cyprus, Denmark, Finland and Malta, are not included due to missing availability of operational urban and sub-urban background monitoring stations in the Urban Audit cities.